This invention relates generally to novel catalysts for use in the epoxidation of olefins, having particular utility in the epoxidation of propylene to give propylene oxide. The catalyst is prepared via precipitation and is a lanthanide-promoted, supported silver catalyst. The invention also relates to methods for catalyzing epoxidation reactions using the catalysts, and to methods for manufacturing the catalysts.
Although epoxidation of ethylene has been industrially applied for many years, the catalysts used typically show poor selectivity in the epoxidation of propylene. As a consequence, direct oxidation of propylene to propylene oxide with high selectivity and activity has yet to be achieved in heterogeneous catalysis.
Catalysts currently used in ethylene epoxidation often contain reduced silver and an xcex1-alumina carrier. Catalysts of this nature have been disclosed, for example, in U.S. Pat. No. 4,248,740 to Mitsuhata et al. and U.S. Pat. No. 4,342,667 to Armstrong et al. Other ethylene epoxidation systems have been developed using catalysts that include small amounts of alkali metals such as potassium, sodium, rubidium, and cesium that act as promoters when used in conjunction with reduced silver. U.S. Pat. No. 4,010,115 to Nielsen et al. describes such a system. A review of the direct epoxidation of ethylene in the presence of supported silver catalysts is provided by Sachtler et al. (1981) Catalysis Reviews: Science and Engineering 23 (1and2): 127-149.
Unfortunately, these ethylene oxidation catalysts display low selectivity in the epoxidation of propylene. While not wishing to be bound by theory, the inventors propose that the low selectivity of these catalysts can be explained by the mechanism for the formation of propylene oxide. The reaction appears to involve oxygen radicals on the surface of the silver which interact with the propylene to form an epoxide intermediate. xcex1-Hydrogen atoms are then removed from the intermediate by a neighboring oxygen atom, resulting in the formation of CO2 rather than propylene oxide.
Attempts to improve the selectivity of propylene epoxidation catalysts have typically focused on the preparation of silver sites without the destructive neighboring oxygen radicals. The isolation of silver sites has been attempted by preparing catalysts having very small silver particles, using either zeolites, as in Giordane et al. (1981) Z. Phys. Chem. Neue Folge 127:109 and Jacobs et al. (1979) J. Chem. Soc. Faraday I 75:59, sol-gel methods, as in Breitscheidel et al. (1991) Chem. Mater. 3:559, or alloying of the silver with gold, as in Geenen et al. (1982) J. Catal. 77:499 and Toreis et al. (1987) J. Catal. 108:161.
Another approach to controlling the proximity of active sites is based on the idea of poisoning a specific fraction of the silver sites with either an alkali metal or a gaseous additive such as CO2. Examples of this type of pretreatment are discussed in U.S. Pat. Nos. 5,856,534 and 5,780,657 to Cooker et al. Poisoning with alkyl halides has also been successfully used to improve the selectivity of propylene epoxidation catalysts, as disclosed, for example, in U. S. Pat. No. 5,770,746 to Cooker.
Modifications to the catalyst support have also been carried out in an effort to improve selectivity. Canadian Patent No. 1,282,772 describes the use of an alkaline earth metal carbonate as the exclusive support material for silver in an epoxidation catalyst for ethylene, propylene and other olefins. This catalyst does not contain any of the traditional support materials, e.g., alumina, and consequently the amount of alkaline earth metal carbonate that must be used is significantly higher than previously disclosed in the art. This catalyst system uses KNO3 in combination with NO/NO2 and an alkyl chloride as gas phase additives.
Various modifications of the catalyst system described in the aforementioned Canadian patent have been recently made using potassium salts and molybdenum promoters (see U.S. Pat. No. 5,625,084 to Pitchai et al.), pretreating with high temperature organic chloride and molecular oxygen steam (see U.S. Pat. No. 5,770,746 to Cooker et al.), using inorganic chloride promoters and potassium promoters, (see U.S. Pat. No. 5,780,657 to Cooker et al.), pretreating with CO2 (see U.S. Pat. No. 5,856,534 to Cooker et al.), and using tungsten and rhenium promoters (see U.S. Pat. No. 5,861,519 to Kahn et al. and U.S. Pat. No. 5,864,047 to Gaffney).
The deposition of silver onto the carrier can be achieved by a number of techniques. One technique that is frequently employed involves the impregnation of the support with a silver solution followed by heat treatment of the impregnated support, to effect deposition of the silver on the support. Another common technique involves coating the silver on the support by precipitating the silver, or a preformation of silver, into a slurry and placing the support in the slurry. The slurry is then heated to remove the liquids present. As the liquids are removed, the silver particles are deposited on the support and adhere to the support surface.
Thus, the art provides propylene oxidation catalysts using alkali earth metal carbonate supports in conjunction with alkali, halogen, tungsten, and rhenium promoters and in conjunction with CO2, NO/NO2 and/or an alkali metal chloride as a gas phase additive. It has now been unexpectedly discovered that a catalyst that is highly selective for the direct production of propylene oxide from propylene is obtained by using an alkaline earth metal carbonate as a support in combination with a rare earth metal promoter. Also surprising is the finding that such catalysts are capable of selective propylene oxidation in the absence of the gaseous CO2, NO/NO2 and/or an alkyl chloride promoter that have heretofore been commonly used to improve epoxide selectivity in vapor phase processes of this type.
Accordingly, it is a primary object of the invention to provide a process for the conversion of propylene to propylene oxide having increased selectivity for propylene oxide and a higher conversion of propylene than have been obtained previously.
It is a further object of the invention to provide a composition useful as an epoxidation catalyst for use in the conversion of propylene to propylene oxide having increased selectivity for propylene oxide and a higher conversion of propylene than have been reported with the prior art.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.
In a first embodiment, then, a novel process for the conversion of propylene to propylene oxide is provided wherein a propylene and oxygen-containing gas feedstream contacts an alkaline earth metal carbonate-supported silver catalyst that comprises a catalytically effective amount of silver and a promoting amount of a rare earth metal promoter.
In a second embodiment, a process for the conversion of propylene to propylene oxide is provided wherein a propylene and oxygen-containing gas feedstream contacts an alkaline earth metal carbonate supported silver catalyst that has a catalytically effective amount of silver, and a promoting amount of a rare earth metal, a halogen anion, and an alkali metal nitrate.
In a further embodiment, catalyst compositions for the conversion of propylene to propylene oxide are provided having an alkaline earth metal carbonate support, a catalytically effective amount of silver, and promoting amounts of a rare earth metal promoter.
In a still further embodiment, catalyst compositions for the conversion of propylene to propylene oxide are provided that have an alkaline earth metal carbonate support, a catalytically effective amount of silver, a promoting amount of a rare earth metal, a halogen anion, and an alkali metal nitrate.
In yet another embodiment, methods for making a catalyst composition for the conversion of propylene to propylene oxide are provided, comprising the steps of: (a) preparing a basic solution comprising an alkali metal carbonate and an alkali metal hydroxide; (b) preparing a precursor solution comprising an alkaline earth metal salt, a silver salt and a rare earth metal salt; and (c) mixing the basic solution with the precursor solution, thereby precipitating the catalyst composition.